<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>02_CSS选择器</title>
    <style>
        div {
            color: red;
        }
        #aa {
            color: green;
        }
        .success{
            background-color: green;
            color: white;
        }

        .error{
            background-color: red;
            color: white;
        }

    </style>
</head>
<body>
<div>我是div</div>
<div>我是div</div>
<p id="aa">我是段落</p>
<p>我是段落</p>
<h2>我是标题</h2>
<h2>我是标题</h2>
<br>
<hr>
<span class="error">用户名错误</span>
<span class="error">密码错误</span>
<span class="success">用户名正确</span>
<span class="success">密码正确</span>
<hr>
<a href="https://www.baidu.com">百度</a>
<a href="123">超链接</a>
</body>
</html>